This invention relates to a pumping system useful in dispensing fluids, especially those which are expensive, viscous, high purity, and/or sensitive to molecular shear.
The invention has numerous applications, but is especially useful in the microelectronics industry. The trend in that industry is to squeeze greater quantities of circuitry onto smaller substrates. Circuit geometries have been shrunk to less than one micron. In that microscopic world, the slightest particle of contamination can create a defect, decreasing production yields, degrading device performance, and reducing device reliability.
For this and other reasons, modern manufacturing techniques in the microelectronics and other industries sometimes involve decontaminated "cleanroom" environments. Many of these techniques also use advanced process chemicals, some of which are very expensive. For example, certain chemicals used to process semiconductors can cost $15,000 or more per gallon, and the semiconductor substrates can be worth $20,000 or more at that stage of processing.
To be useful in cleanroom environments and applications, however, the chemicals must be filtered. Because of the viscosities and sensitivities of the fluids, they must be filtered at low flow rates and under low pressure to minimize molecular shear on the fluids. Prior art devices do not meet these parameters in certain production-line operations.
For example, some operations require a periodic, non-continuous "shot" of fluid. Such "shots" sometimes consume only a small part of the pump's cycle time, leaving the pump and/or filter idle during the remainder of the cycle. During that relatively brief moment when a shot occurs, high pressure must be used to achieve a flow rate sufficient to dispense an appropriate amount of fluid. As noted above, such high pressures and flow rates can damage sensitive fluids.
In addition, low pressure filtration is generally recognized as the best way to effectively eliminate gel slugs in, and remove contaminants from, a subject fluid. If high pressure is used to achieve a desired flow rate through a filter, contaminants can be forced through the filter, rather than retained therein.
Furthermore, many operations, especially in the semiconductor industry, apply only small amounts of fluid to each unit processed. In these applications, there is an increased need for precise control over the dispense.
Additionally, the reservoir of subject fluid needs to be easily monitored, replaced, and/or replenished. These dispense systems also need to be easily primed with and purged of subject fluid, to allow the system to be used on more than one fluid, and to reduce fluid shear.
At the present time there is no system that satisfactorily meets these various requirements. In fact, in some research laboratories, these expensive fluids are still being dispensed by hand; that is, lab technicians or scientists pour the fluids directly out of storage containers. This hand pouring has poor repeatability, involves significant operator technique, does not allow point-of use filtration, and generally causes a tremendous, expensive waste of time and materials. Production and laboratory costs could be greatly reduced by automating the dispense of these fluids.
Numerous other problems exist with prior art dispense systems. In certain operations where relatively high pressure is acceptable and desired to achieve a necessary flow rate, such as through a filter which is still useful even though partially clogged, prior art systems cannot deliver, or are inaccurate when delivering, the required pressure. The systems have poor predictability and repeatability of results. Their complicated flowpaths are difficult to purge, and excessive fluid hold-up volumes lead to fluid waste.
Prior art systems also waste fluid during dispensing and provide little, if any, in the way of "suck-back" adjustment. Suck-back is an adjustment made at the outlet port of a given dispense system, in which the fluid is drawn back slightly inside the port. This adjustment reduces fluid solvent evaporation at the outlet during idle periods, reduces fluid contamination at the outlet, and most importantly allows for a sharp and dripless cessation of dispense, avoiding waste of the processed fluid.
Additionally, prior art systems are not easily automated, their fluid reservoir levels cannot be easily monitored, and they are limited in the range of fluid viscosities which they can dispense. Finally, complex mechanisms downstream of the filter often generate fluid contaminants.
For example, certain prior art systems utilize diaphragm-type pumps in which the diaphragm is actuated by air pressure. Typically, the actuating air is more compressible than the liquids being pumped. As air pressure is increased in an attempt to displace the diaphragm and dispense fluid, the actuating air is compressed, in effect "absorbing" part of the intended displacement of the diaphragm. This air compression prevents accurate control and monitoring of the position of the diaphragm and, correspondingly, prevents accurate control and monitoring of the volume and rate of fluid dispensed.
The problem is exacerbated if the fluid is being pumped through a filter. By its nature, the filter becomes clogged during use. As it becomes clogged, higher pressure is required to achieve a given flow rate through the filter. Because the air pressure actuating the diaphragm typically remains relatively constant throughout the life of the filter, however, fluid flow rate through the filter decreases as the filter becomes more clogged, making it even more difficult to achieve repeatable, accurate dispense.